1. Field of the Invention
This invention relates generally to forming solder connections on a circuitized substrate and, more particularly, to a method of forming solder connections on a circuitized substrate having connection pads and circuitry on the substrate, with a solder mask formed of laser ablatable material overlying the pads, and dispensing liquid solder onto the pads.
2. Background of the Invention
Mounting of integrated circuit chips (I/C chips) on circuitized substrates is well known in the art. This mounting takes several forms. In one form, the circuitized substrate is provided having contact pads thereon which connect to the remainder of the circuitization on the substrate. The I/C chips are mounted to the pads by flip chip bonding, using C4 solder joints. A solder mask is applied over the pads and the substrate and, in one embodiment, the solder mask is in the form of a photopatternable or photoimageable epoxy. After the solder mask has been photoimaged and developed to form openings in the solder mask overlying the pads, solder is applied by various techniques to the pads revealed by the formed openings. This form, while appropriate for many applications, has certain limitations when used for certain other applications. First, it is difficult to define small features and pitches precisely by photoimaging. Moreover, the uneven exposed surface of the solder masks, coupled with the variation in size of the photoformed openings, makes it difficult to apply a precise amount of solder to the underlying pads, especially in circuitized substrates having very small features and pitches of lines and pads. Therefore, it is desirable to have a technique of applying solder to the pads on a circuitized substrate wherein a precise amount of solder can be applied and which is applicable to fine line and fine pitch technology.
According to the present invention, a method of forming solder connections on a circuitized substrate is provided. This method includes the steps of providing a circuitized substrate, which circuitization includes a plurality of connection pads. A laser ablatable solder mask material is degassed and then dispensed preferably in paste form onto the substrate over the circuitization; preferably, the solder mask material is an epoxy. The surface of the solder mask material as applied is leveled, preferably with a PTFE coated glass plate, and the solder mask material is then cured to form a solder mask. Openings are laser ablated in the solder mask material to reveal those connection pads which are to receive solder to form the solder connections. Liquid solder is dispensed under pressure in a confined space into the openings as blades move laterally on top of the solder mask to fill the openings to the height of the openings in the solder mask. The solder material is then solidified to form domed solder bumps in the openings extending above the solder mask material.